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Who Really Needs a Continuous Glucose Monitor?

The wearable tech is proven effective for diabetes management but not for prevention, doctors say


spinner image man working in a recording studio wearing a continuous glucose monitor on his upper arm
For most older adults, it’s easy to wear a continuous glucose monitor during daily activities.
Courtesy Dexcom

If there’s one thing Chih Long Liu knows for sure, it’s that he doesn’t want to suffer the same fate his mother endured. A senior research scientist at Stanford University, Liu relies on data to offset chance — and he’s taking that same studied approach to prevent diabetes. Liu’s mother died after developing end-stage kidney failure, a complication of type 2 diabetes, and her passing had a profound impact on Liu and his sister.

“Of course that made us very cognizant of the potential familial risk factors, and we wanted to try to stave off type 2 diabetes in ourselves for as long as possible,” says Liu, who is 44 and lives in Saratoga, California.

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About six years ago, his sister started wearing a continuous glucose monitor, which allowed her to check her glucose levels throughout the day. Then about a year after that, Liu also began using a CGM after learning he had elevated blood sugar, or prediabetes — a precursor to diabetes — in an effort to bring his blood sugar back into a healthy range.

Liu says the CGM helped him to notice patterns, such as how eating particular foods spiked his glucose levels, that helped him get it under better control.

“We would certainly credit the CGM sensors [for] contributing significantly toward … modifying diet and exercise to help us achieve that goal,” Liu says.

How does continuous glucose monitoring work?

CGMs have a tiny sensor that’s inserted under the skin typically through the arm or abdomen. “It’s not quite as uncomfortable as getting a shot,” Liu shares. “It’s almost more like a pinprick.” Thereafter, the wearable devices avoid the need for pricking a finger several times daily, as required with traditional glucose meters.

Liu wears his device continuously for 14 days before taking it off and leaving it off for a day, then puts the CGM back on for another two-week stretch. By comparison, people with diabetes — particularly type 1 — usually wear it continuously; others who don’t have diabetes may wear it only for a finite period to make lifestyle adjustments.

Instead of blood sugar, CGM sensors measure the glucose in what’s called interstitial fluid, which is found in the space between skin and muscle where there’s fat tissue. Readings are typically taken every few minutes and accessed via a reader or smartphone. In addition to the real-time information, CGMs may also alert users with an audible alarm or beeping if their glucose levels have dropped below or gone above a set range.

More data needed to support CGM for diabetes prevention

While health experts are excited by the prospect of a data-driven approach to preventing diabetes — which affects more than 37 million Americans — they say CGM’s effectiveness for anything outside diabetes management hasn’t yet been proven.

What’s really needed is a randomized trial where a CGM is used by some participants and not by others to show whether fewer who used the device go on to develop diabetes, emphasizes Vivian Fonseca, M.D., chief of endocrinology and the Tullis Tulane Alumni Chair in Diabetes at Tulane University School of Medicine in New Orleans. “There is no such study,” he says.

The devices, which require a prescription, are FDA-approved for managing all types of diabetes, and the vast majority are still used for just that, according to device makers and distributors. However, as people become increasingly interested in tracking their vitals and other personal health data — like using smartwatches to monitor heart rate and sleep quality — some who don’t have diabetes are accessing the devices via “off-label” prescriptions. And the technology is already available over-the-counter in Europe.

Companies like Nutrisense, Levels and January AI provide CGM combined with other wellness offerings that include help interpreting results and applying what’s learned to personalize lifestyle choices. Heather Davis, registered dietitian and health content technical writer at Nutrisense, says in most cases CGM is not covered for people who don’t have diabetes, though it’s worth checking with your insurance.

Since first donning his CGM, Liu has participated in precision diet and CGM research conducted by his colleague Michael Snyder, director of the Center for Genomics and Personalized Medicine at Stanford University, and cofounder of January AI. But Liu says he has no financial ties to January or other companies that provide CGMs. 

Snyder cofounded January AI with business executive Noosheen Hashemi in 2017 after finding CGMs helped closely track blood sugar swings that are linked with an increased risk for diabetes and other issues like cardiovascular disease.

Today, even as physicians reiterate more data is still needed to prove its effectiveness outside diabetes management, CGM is being used by everyone from elite athletes hoping a tighter blood sugar range will help with performance to people trying to lose weight. Liu says he’s lost weight and kept off 25 pounds through strict carb control and routine exercise.

“The CGM sensor was very instrumental in helping me identify ... both the kinds of foods that produced significant glucose spikes as well as foods that didn’t,” Liu says, with the former including “hidden carbs” like marinated meats.

False alarms

Liu wears his FreeStyle Libre 2 system by Abbott — one of two major manufacturers that make CGMs in the United States, along with Dexcom — on the underside of his left arm. It has strong adhesive and is compact, allowing him to do everything from lifting weights to showering (it’s water resistant) without taking it off.

In the five years he’s used it, Liu says he hasn’t experienced any side effects after first getting used to having it on. But some doctors worry people who use a CGM on their own may struggle with the results.

“I think where the big gray zone is dispensing these sophisticated technology” without necessarily having a health care provider to interpret the results, says Archana Sadhu, M.D., an endocrinologist and director of the diabetes program at Houston Methodist Hospital.

Technical glitches can also be a concerning factor. Once when Sadhu used a CGM herself to see how her blood sugar fluctuates day-to-day, the device’s alarm went off for nearly 36 hours continuously telling her she had low blood sugar, though she says she had no symptoms. “I confirmed with a blood glucose meter, and it was not low,” Sadhu says. “But the CGM continued to show low.”

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One problem is the range that may be set on a CGM is conducive to blood sugar control for a person with diabetes — but not for someone without it, says Morgan Jones, M.D., inpatient clinical director in the division of endocrinology and metabolism at the University of North Carolina-Chapel Hill.

“In someone who doesn’t have diabetes, those alarms can actually just be harmful to their mental health,” Jones contends. So, for instance, a person might have a blood sugar that is 65 milligrams per deciliter overnight, which is a totally normal blood sugar for someone without diabetes, he says. “Then they wake up in the middle of the night, they have a lot of stress from that alarm [going] off and they have interrupted sleep. On top of that, they may think they have a new medical condition when they really don’t.”

Setting parameters

Even the numbers used for those CGM parameters are disputed.

In Nutrisense’s app, its default glucose level range is set to between 70 and 140, says Davis. “We have evidence to support that parameter for nondiabetics, she says — like an American Diabetes Association expert group’s suggestion that the low threshold be set at 70. But she acknowledges there is some variation in what researchers might consider hypoglycemia, or low blood sugar, and she adds that members are able to customize that range.

spinner image woman wearing a continuous glucose monitor and using her mobile app to scan a nutrition label for glucose content
The Nutrisense app syncs with a CGM to reflect how different foods, stress and more impact the body.
Courtesy Nutrisense

Slight fluctuations in blood sugar levels are normal, and also happen regularly in people who don’t have diabetes, depending on what food they eat, according to the National Institutes of Health. Between around 60 and 140 milligrams of sugar per deciliter of blood (mg/dl) is considered a healthy level.

Davis says instead of automatic alarms, members can get “optional notifications” alerting them to patterns tied to, for example, their glucose numbers after a meal or for the day.

Davis says some consumers come to the company with the idea that their blood sugar range should be even tighter, say between 70 and 120, not allowing for normal swings in blood sugar. A fasting blood sugar level of 100 to 125 milligrams per deciliter is considered prediabetes and 126 mg/dL or above is diabetes. (This is measured after a person abstains from eating for at least eight hours.) She says the organization’s trained nutritionists try to educate people about what’s a healthy range, and that advice goes beyond blood sugar. According to the company’s website, that includes providing expert insights, support and accountability to reach a person’s health goals. 

Are CGMs safe?

In April, more than 4 million FreeStyle Libre glucose readers were recalled by Abbott because the lithium-ion batteries used in the readers could get extremely hot, spark or even catch fire, according to the FDA. The readers were linked to at least 206 incidents, including seven fires and one injury, though no deaths have been reported.

Generally speaking, however, Jones emphasized that the devices are “extremely safe.”

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Personalized prevention

Davis emphasizes CGMs are but one tool to better understand metabolic health.

“One thing that we do emphasize is that we’re not just a CGM company,” she says. “We are a metabolic health company.”

There are many factors that conspire to put one at risk for what’s collectively called metabolic syndrome, or insulin resistance syndrome. That can range from a large waistline (abdominal obesity) to high blood pressure to high triglycerides.

But where 96 million Americans have prediabetes, and the vast majority don’t know it, according to the CDC, many are overlooking a major risk factor — high blood sugar — and missing out on a chance to prevent diabetes. Without timely intervention, up to 7 in 10 people with prediabetes will go on to develop full-blown diabetes, according to an American Diabetes Association expert panel.

“I do think the diabetes endemic is worse than the COVID pandemic, and all you have to do is look at the numbers: 9 percent of people are diabetic, 33 percent of people are prediabetic,” Snyder says. “People don’t really know who’s at risk.”

He argues that CGM provides a unique way to personalize behavior modification, where different foods and other factors ranging from sleep loss affect stress in different ways.

January AI uses artificial intelligence to draw on its database of millions of foods and make recommendations. “Well, if you like this, and it spikes [your blood sugar], eat that instead — because we know that won’t spike you,” Snyder says.

At the same time, device makers are walking the line — not promoting a CGM as diagnostic or curative, which would run afoul of regulations.

“Let me put a disclaimer here: This is an off-label use of the device,” says Tomas Walker, vice president of clinical affairs for Dexcom. “The device’s FDA label says they’re to be used for people living with diabetes.”

But Walker says that doesn’t mean it will stay that way.

“People wear devices to get health information; and it was very natural, I think looking at this in retrospect, that a CGM would be seen as a tool for many people who want to improve their overall health.”

Back to basics

For all the promise of big data, diabetes experts say what’s still sorely needed is for people to see doctors annually and get blood sugar screened. The American Diabetes Association recommends everyone should have their blood sugar tested starting at age 35, and earlier for those with risk factors like a family history of diabetes.

“There are a lot of people who never go to the doctor,” Fonseca says. Access issues like insurance and economic barriers notwithstanding, that’s a bar that’s far less expensive and easier to clear than accessing CGM. Then, once you see your doctor and have your blood sugar checked, “follow the advice,” Fonseca stresses. Even if that’s easier said than done — with or without technological assistance.

Editor's note: This story, originally published June 5, 2023, has been updated.

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